Polymer membranes have been utilized for various separations including gas separation as well as liquid separation. Membrane-based gas separation has become an important alternative to well-established separation operations, such as cryogenic distillation, and adsorption processes. Membrane-based gas separation is a pressure-driven process that does not require a high energy cost phase change of the feed gas mixture, as in other separation operations. Moreover, the mechanical simplicity and small footprint of membrane-based gas separation units provides a great deal of flexibility in installation and operation.
Such advantages have led to a wide range of applications for membrane-based gas separations. These separations include the gas pair (i.e., mixtures of at least two gases to be separated): O2/N2, H2/N2, H2/CH4, CO2/CH4, H2O/air, He/air, He/N2, He/CH4, He/H2, He/CO2, H2/CO2, H2S/natural gas and H2O/natural gas. With increasing costs of energy and environmental concerns regarding CO2 separation, collection and sequestration, gas membrane separation offers significant promise in present and emerging industries. One emerging environmental application could involve membrane CO2/N2 separation of flue gas to allow for CO2 collection and sequestration.
The choice of a membrane material for gas separation applications is based on specific physical and chemical properties, since these materials should be tailored in an advanced way to separate particular gas mixtures. Commercial gas separation modules generally employ organic polymers as asymmetric non-porous membranes. The polymeric membrane materials are typically used in processes in which a feed gas mixture contacts the upstream side of the membrane, resulting in a permeate mixture on the downstream side of the membrane with a greater mole fraction of one of the components than the composition of the original feed gas mixture. A pressure differential is maintained between the upstream and downstream sides, providing the driving force for permeation. The downstream side can be maintained as a vacuum, or at any pressure below the upstream pressure.
The membrane performance is characterized by permeability and selectivity. Permeability (P) is the rate at which any gas component permeates through the membrane. The separation of a gas mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity. Selectivity (S) can be defined as the ratio of the permeabilities of the gas components across the membrane. The selectivity is a key parameter to achieve high product purity at high recoveries. A membrane's permeability and selectivity are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent. It is desired to develop membrane materials with a high selectivity (efficiency) for the desired component, while maintaining a high permeability (productivity) for the desired component.
Typically, polymeric membranes show high selectivity and low permeability (throughput) when compared to porous materials, due to their low free volume. Polymer free volume, the fraction of the volume not occupied by the electronic clouds of the polymer, plays an important role in the transport properties of low molecular weight species and gases.
An amorphous polymer is in a rubbery state above its glass transition temperature (Tg). It presents a relatively large amount of free volume, owing to transient voids between the highly mobile polymer chains. When the temperature is lowered below its Tg, the polymer is in a glassy state, and behaves like a rigid glass: the fractional free volume decreases, resulting in insufficient space for large-scale co-operative movements of the polymer backbone.
Glassy polymers are differentiated from rubbery polymers by the rate of segmental movement of polymer chains. Polymers in the glassy state do not have the rapid molecular motion that permit rubbery polymers their liquid-like nature and their ability to adjust segmental configurations rapidly over larger than 0.5 nm distances. Glassy polymers exist in a non-equilibrium state with entangled molecular chains with immobile molecular backbones in frozen conformations. Generally, glassy polymers provide a selective environment for gas diffusion and are favored for gas separation applications. Rigid, glassy polymers are preferred as polymers with rigid polymer chain backbones that have limited intramolecular rotational mobility and are often characterized by having a high glass transition over 100 degrees C.
Almost all industrial gas separation membrane processes utilize glassy polymers because of high gas selectivity and good mechanical properties. In glassy polymers, the more permeable species are those with low molecular diameter and selectivity is due to differences in molecular dimension. The glassy state is characterized by a relatively small fraction of free volume. A larger amount of free volume (up to 20%) can be “frozen-in” by rapid cooling or by a rapid removal of a solvent in some polymers with stiff molecular structures. Free volume is locked into the structure as molecular mobility does not allow relaxation to fill the void space created with decreasing temperatures. The excess free volume is considered a non-equilibrium situation that is kinetically prevented from reaching an equilibrium condition due to the restriction of movement of polymer chains below the glass transition temperature. Medium to high free volume glassy polymers (e.g., polyimides, polyphenyleneoxides, poly(trimethylsilylpropyne), etc.) are used to produce membranes since the voids aid the transport of gas or liquid through the material.
In addition to the overall amount of free volume, polymer properties are also influenced by the distribution of micropores, particularly when the free volume elements are interconnected. Polymeric membranes generally undergo a trade-off limitation between permeability and selectivity: as selectivity increases, permeability decreases, and vice versa. Robeson showed in several references (L. M. Robeson, J. Membr. Sci. 62, 195 (1991); B. D. Freeman, Macromolecules 32, 375 (1999); L. M. Robeson, J. Membr. Sci. 320, 375 (2008)) that as for small gaseous molecules (e.g., O2, N2, CO2, and CH4) a superior limit or “upper bound” exists in a selectivity/permeability diagram. To achieve higher selectivity/permeability combinations, materials that do not obey those simple rules would be required.
A recent publication has noted that the upper bound can be exceeded with a polymer system that is thermally rearranged to promote main chain heterocyclic structures not present in the precursor polymer (Park et al., Science 318, 254 (2007)). It was noted that the pore size distribution in the thermally rearranged polymer is much narrower than in the precursor polymer, yielding molecular sieving like permeability/selectivity properties. Increasing free volume leads to increased permeability and decreasing the pore size distribution in polymers leads to increased selectivity. Methods to achieve both simultaneously are highly desired.
Creation of voids in polymer systems has been noted. Methods include selective decomposition of a thermally labile block from a block copolymer, or a thermally unstable component from a polymer blend, or an added porogen during polymerization. However, all of these methods produce porous polymers but with pore sizes well above the size desired for gas separation. Porogens yielding molecular scale dimensions are much less common.
Decomposition of pendant labile groups has been reported to increase free volume of polymers. Zhou et al. (Chem. Lett. 2002, 534) and Islam et al. (H, Mem. Sci. 2005, 261, 17) reported the thermal decomposition of pendant sulfonic acid groups of polyimide to increase free volume of the polymer membrane. The decomposition of sulfonic acid induced microvoids and led to increased free volume. Increased free volume enhanced gas permeability. The thermal decomposition was carried out at the temperatures higher than the glass transition temperatures of the polymers, which resulted in the relaxation of the polymer chains and the collapse of some of the microvoids created.
Despite the foregoing developments, there is still room in the membrane separation art for further improvements.
Thus, in the design of polymeric membranes for gas separation, it is desired to increase free volume by providing pore sizes having a narrower distribution than that typically achieved with solution casting or melt processing of polymers.
It is therefore desired to provide a polymer with increased free volume.
It is further desired to provide a method for producing a polymer with increased free volume.
It is still further desired to provide a gas separation membrane produced from a polymer with increased free volume.
It is still further desired to provide a process for producing a gas separation membrane produced from a polymer with increased free volume.
All references cited herein are incorporated herein in their entireties